Diagnostic tests have become an indispensable tool of a physician's armamentarium. Accompanying many diseases and conditions are subtle changes in the body's physiology, particularly in concentrations of compounds associated with the condition. The catecholamines are a group of these compounds. They are identified by the catechol nucleus ##STR1## wherein R is the amine function of the particular compound. The significant catecholamines found naturally in the body are epinephrine, R is ##STR2## hereafter referred to as E; norepinephrine, R is ##STR3## hereafter referred to as NE: and dopamine, R is --CH.sub.2 CH.sub.2 NH.sub.2, hereafter referred to as DA. These catecholamines are found in various tissues of the body, including, inter alia, blood, urine, cerebrospinal fluid, and brain tissue. The quantities of these compounds are extremely small. For example, in blood serum E and NE are present in picogram (10.sup.-12) quantities.
In recent years there has been substantial interest in measuring catecholamine levels in mammalian systems. Catecholamine levels are significantly increased when pheochromocytomas are present. The catecholamine levels are influenced by the presence of other tumors, such as neuroblastoma, which affect the central nervous system. Knowledge of catecholamine levels is of significance in the diagnosis and management of hypertension, coronary disease, angina pectoris, acute myocardial infarction, and diabetes mellitus.
An early method for measuring catecholamine levels in mammalian systems which was sufficiently sensitive to reach the submicrogram range was the spectrofluorimetric assay of Von Euler and Floding (Acta Physiol. Scand. 33: Suppl. 118, 45, 1955). This assay was based on the oxidation of NE and/or E with ferricyanide or iodine to the corresponding trihydroxyindole which in alkaline solution yields a highly fluorescent noradrenolutine or adrenolutine. NE or E must be first isolated from the mammalian system, for example, blood serum or plasma, the supernatant of deroteinized tissue homogenate or cerebrospinal fluid, before the assay can be done. DA is oxidized by iodine to form a dihydroxyindole. The fluorophore produced by this derivative is different from that of NE or E and is the basis for the fluorimetric assay (Carlson and Waldeck, Acta. Physiol. Scand. 44: 293, 1958). As with NE and E, DA must also be isolated from the mammalian system prior to the assay.
Isolation of the catecholamines from the supernatant of deproteinized tissue homogenate, the blood plasma or the biological fluid is generally done by one of two techniques. The first and most widely used isolation technique is the adsorption of catecholamines onto neutral alumina at pH 8.4-8.6. Catechol containing compounds are adsorbed by alumina and noncatechol compounds from the mixture are discarded. Catechols are removed from the alumina by acid elution.
The second isolation technique employs cation exchange chromatography. Due to the ionization of these amines at acidic pH, NE, E and DA can be bound to the exchange resin and then eluted preferentially by increasing acid strength.
Both of these isolation processes are quite lengthy and generally do not yield fractions which are specific for a particular catecholamine. Moreover, the spectrofluorimetric assay systems are relatively insensitive for the needs of the investigator. For NE and E a sensitivity of approximately 50-500 nanograms is obtained. Although refinement of this method has led to techniques that are sensitive to quantities as small as 2 to 3 nanograms, the accuracy of assays at this level of sensitivity are poor, see Engelman et al. Am. J. Med. Sci., 255, 259 (1962). The sensitivity of DA is approximately 500 nanograms. Even though this assay is relatively insensitive, all the commercial laboratories but one use the spectrofluorimetric assay or some adaptation of it for measuring catecholamines.
Another method of commercially measuring catecholamine levels is potentially available. The enzyme catechol-O-methyl transferase, initially reported and characterized by Axelrod and Tomchick, J. Biol. Chem., 233, 702, 1958, and hereafter referred to as COMT, is an enzyme which transfers a methyl group from a donor molecule to a catechol nucleus thereby forming a 3-methoxy moiety depicted below in FIG. 2 wherein R is the amine functionality. ##STR4##
Later investigators using radiotracers and other techniques including chromatography have related the radiolabeled quantities of metanephrine, hereafter referred to as MN, normetanephrine, hereafter referred to as NMN and methoxytyramine, hereafter referred to as MEOT to the initial quantities of E, NE and DA in the mammalian system. The chemical structures of MN, NMN, and MEOT are shown in FIG. 3. ##STR5##
In 1968 Engelman, et al., Am. J. Med. Sci. 255, 259 (1968) utilized the double isotope dilution derivative technique with COMT to assay for catecholamines, i.e., epinephrine and norepinephrine, in biological specimens. An internal tracer, 7-H.sup.3 -norepinephrine was employed with the methyl donor S-adenosyl-L-methionine-methyl-.sup.14 C. Prior to assay the urine and blood samples were pretreated to separate the catecholamines. The urine was chromatographed over alumina and the blood sample chromatographed over a cation-exchange resin. After incubation of the chromatographed samples in the enzymatic system, the incubation was stopped and the resulting solution treated and chromatographed over a cation exchange resin. The metanephrines were eluted and oxidized to vanillin with periodate. The vanillin was extracted, purified and counted in a liquid scintillation medium. The typical complex double isotope measurement and calculation was required to find the quantity of catecholamine present.
Engelman followed up his earlier work with another publication, Engelman et al., Circulation Research, 26, 53 (1970). Modification of his earlier work now allowed the differential measurement of E and NE in the same sample of 5 to 10 ml. human blood plasma. The same double isotope technique was employed as in the previous publication except that the MN and NMN are separated with thin layer chromatography. Tritium labeled tracer quantities of both E and NE were added along with 100 .mu.g. of both MN and NMN. Prior to assaying, both plasma and urine samples were chromatographed over cation exchange resins. Ethylenediaminetetraacetic acid was added to the plasma sample prior to the chromatographic step. The sensitivity of the assay, as measured by the value which is twice that of the blank, was 250 picograms. Plasma E values cannot be measured very accurately at the lower end of the normal range.
In Passon and Peuler, Analytical Biochemistry, 51, 618, ( 1973) E and NE were assayed using COMT, followed by separation of MN and NMN by chromatography, and thereafter oxidation to vanillin. The blood serum or plasma was used directly without any prior chromatography. Only a single isotope, S-adenosyl-methionine(.sup.3 H)methyl was used. The tritiated methyl donor was diluted with cold methyl donor. Tracer labeled .sup.14 C compounds were tried but were not helpful in reducing problems such as an accurate estimate of recovery of the catecholamine substrate. At concentrations below 10 .mu.M of cold S-adenosylmethionine, the blank increased substantially. Assay techniques were otherwise similar to those used by Engelman, et al. and others. The sensitivity of the assay was 170 pg. for NE and E, thereby allegedly allowing the measurement of NE and E in less than 1 ml. of human serum.
Coyle and Henry, J. of Neurochemistry, 21, 61 (1973) separated norepinephrine from dopamine in deproteinized brain tissue homogenates by using COMT and S-adenosyl-methionine(.sup.3 H)methyl. After the addition of non-radioactive carrier quantities of NMN and MEOT, the incubate was extracted by an organic solution and the extract repartitioned into aqueous hydrochloric acid. The aqueous phase was washed with the organic solvent. Vanillin-(.sup.3 H) was produced by periodate cleavage .sup.3 H NMN. The vanillin-(.sup.3 H) was extracted into an organic solvent and eventually counted in a liquid scintillation medium. The MEOT was extracted from the aqueous periodate phase with a borate buffer and a 3:2 (v/v) toluene:isoamyl alcohol solvent system and counted with a liquid scintillator.
Christensen, Scand. J. Clin. Lab. Invest. 31, 343 (1973) determined the level of DA in plasma by a double isotope technique utilizing the COMT procedure. Prior to the enzymatic methyl transfer, the catecholamines were isolated from plasma utilizing aluminum. The COMT and the sample were incubated with S-adenosylmethioninemethyl (.sup.14 C) and tracer quantities of tritium labeled DA. After oxidation of MN and NMN with periodate to vanillin, the MEOT is separated and analyzed in a liquid scintillator. The DA concentration was calculated on a computer from the counts of .sup.14 C and .sup.3 H.
Cuello et al. in 1973 analyzed deproteinized brain tissue extracts for DA in the presence of NE. COMT and tritium labeled S-adenosylmethionine were mixed with cold S-adenosylmethionine or .sup.14 C S-adenosylmethionine. After extracting the incubate with an organic solvent, the organic solvent is back extracted into hydrochloric acid and then chromatographed.
In 1975, Yamaguchi et al., Circulation Research, 36, 662 (1975) assayed for endogenous catecholamines in blood with ethylene glycol bis-(aminoethyl ether)-N,N'tetraacetic acid (hereafter referred to as EGTA) present in the incubation mixture.
Champlain et al., Circulation Research, 38, 109 (1976) disclosed that calcium ion was known to inhibit the reaction of COMT from work done by Axelrod and Tomchik. EGTA is a selective chelator of calcium as opposed to magnesium and restores the activity of the enzyme. However, a later publication, Weinshilboum, et al., Biochemical Pharmacology, 25, 573 (1976) states that the possibility that other cations might inhibit the COMT reaction in the presence of optimal concentration of magnesium had not been studied to date.
A new system for assaying catecholamines has now been developed, utilizing the basic COMT system of the past. Catecholamines content of mammalian systems can now be assayed to a sensitivity of 5 picograms for NE and E and 12 picograms for DA. The assay is precise and unusually selective. Furthermore, the assay can be performed rapidly by laboratory personnel having minimal advanced technical training. The assay is presently being used on a successful commercial basis by The Laboratory Procedures Division of The Upjohn Company.